To recover valuable fluids from subterranean formations, wells are drilled from the surface of the earth to the productive formations. In the drilling of such wells, a rotating drill bit is commonly employed. As the bit rotates, penetrating through to the formation, material is dislodged in the form of cuttings. These cuttings are commonly removed from the well during the drilling operation by means of a drilling fluid, which may be comprise water, oil, an emulsion of water and oil or foam. The drilling fluid is circulated downward through the drill pipe and upward through the annulus between the drill pipe and the wall of the well, carrying the cuttings with it to the surface of the well in the form of a slurry. The drilling fluid also serves to cool the drill bit and can prevent blow-outs when drilling into strata containing high pressure fluids. When drilling a well, it is common to start with a relatively large diameter hole and cement surface casing in the hole. Subsequent drilling operations are then conducted through this casing. As drilling progresses deeper into the well, the diameter of the hole drilled may be reduced in steps, with progressively smaller diameters of casing employed in response thereto.
In seeking to recover hydrocarbon-bearing fluids from subterranean formations, it is often the case that such fluids are found to reside within formations which are unconsolidated. Unconsolidated formations often comprise poorly cemented sandstone which have little or no cementing material holding the grains of sand together. The production of hydrocarbons from unconsolidated formations often results in the concomitant production of sand. As those skilled in the art readily appreciate, the production of sand is undesirable for many reasons, chief among these being that it is abrasive to the components within the well, such as tubing, pumps, valves and the like, causing rapid erosion of such equipment and, in addition, may result in the partial or complete blockage of the well. Sand production is often rate sensitive, that is, no sand may be produced at very low rates of production, while at higher rates, large amounts of sand may be produced.
A variety of techniques have been employed to control the flow of sand from unconsolidated formations. Many of these techniques employ the use of slotted or screened liners or gravel packs to prevent the sand from being transported along with the hydrocarbons into the well. For example, in the heavy oil sands of California, well completions generally employ slotted liners. Typically, the slotted liner is drilled into the producing zone of the formation with foam, to a predetermined depth. Advantages accruing from the use of foamed-in liner completions include: reduced drilling expense, increased production and the biodegradability typical of such foams. However, these well completions, without being gravel packed across the unconsolidated producing zone, experience higher operational costs due to produced formation sand.
In certain situations, when attempting to install a gravel packed liner in an unconsolidated formation, a variety of problems can be encountered. One such problem arises when attempting to underream a drilled pilot hole with foam for a cleaner fluid prior to gravel packing. As is often the case, when conducting a conventional underreaming operation, the underreamed hole will collapse before the liner is positioned for gravel packing due to the unconsolidated nature of the formation and the fact that the underreaming tool must be removed before the gravel pack is installed.
Underreamers are a type of borehole tool which is used to enlarge a borehole which has already been drilled. In an underreaming operation, an expandable drilling tool is introduced through the casing to the point where underreaming is to be conducted. There, the underreamer is expanded to drill the formation to a larger diameter than the hole through which the underreamer passed. A typical underreamer includes expandable arms mounted in a housing by hinge pins for movement between a closed position and an open, expanded position. In the usual case, the expandable arms are moved outwardly by means of a pressure activated piston mounted within the main bore of the tool's housing. Underreamers come in a variety of types. One type of underreamer employs rotatable cone bits, mounted on the ends of the expandable arms for engaging certain types of formation and is generally referred to as a roller cone underreamer. Another earlier type of underreamer is known as a drag bit underreamer. In the drag bit underreamer, the expandable arms have a machined surface which is typically coated with a hard facing material for engaging and enlarging a borehole after the initial hole has been cut. The machined surface may have diamond bit implants such as those manufactured by General Electric under the trademark "Stratapax". As is known in the art, underreamers may be mounted at the end of the drill string or, in the case of a drilling type underreamer, mounted in the drill string above the drill bit.
Despite these advances in the art, there exists a need for an improved apparatus and method of placing a liner and gravel pack in an unconsolidated formation.